Design of a Full-Stokes Polarimeter for VLT/X-shooter

• Main Authors: Snik, Frans, van Harten, Gerard, Navarro, Ramon, Groot, Paul, Kaper, Lex, de Wijn, Alfred
• Format: Journal Article
• Language: English
• Published: 12-07-2012
• Subjects: Physics - Instrumentation and Methods for Astrophysics; Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.1207.2965

X-shooter is one of the most popular instruments at the VLT, offering
instantaneous spectroscopy from 300 to 2500 nm. We present the design of a
single polarimetric unit at the polarization-free Cassegrain focus that serves
all three spectrograph arms of X-shooter. It consists of a calcite Savart plate
as a polarizing beam-splitter and a rotatable crystal retarder stack as a
"polychromatic modulator". Since even "superachromatic" wave plates have a
wavelength range that is too limited for X-shooter, this novel modulator is
designed to offer close-to-optimal polarimetric efficiencies for all Stokes
parameters at all wavelengths. We analyze the modulator design in terms of its
polarimetric performance, its temperature sensitivity, and its polarized
fringes. Furthermore, we present the optical design of the polarimetric unit.
The X-shooter polarimeter will furnish a myriad of science cases: from
measuring stellar magnetic fields (e.g., Ap stars, white dwarfs, massive stars)
to determining asymmetric structures around young stars and in supernova
explosions.